1. The Field of the Invention
The present invention relates to a method for a radar for determining a noise floor level that is increased in response to interference by a radar wave transmitted from some other radar. The present invention further relates to an interference detecting device for a frequency modulated continuous wave (FMCW) radar and the FMCW radar equipped with interference detecting device using the method for estimating the noise floor level to accurately determine occurrence of interference between the FMCW radar and some other radar.
2. Description of the Prior Art
A number of automotive radar systems which are suited to vehicle safety system, for example, crash protection systems that minimize the effects of an accident, reversing warning systems that warn the driver that the vehicle is about to back into an object such as a child or another vehicle and the like, are known. Hence, it is important for these automotive radar systems to provide the driver with some information as to the nature or location of a target object. One target characteristic of great importance is the distance from the radar to the target object (the downrange distance). In particular, if there are multiple target objects, distances to those target objects are important information for the driver. Thus, it is obvious that radars that provide accurate downrange information for multiple target objects are desired.
The simplest automotive radar systems use a continuous wave (CW) radar in which a transmitter continuously transmits electromagnetic energy at a single frequency. The transmitted electromagnetic energy is reflected by a target object and received by the radar receiver. The received signal is shifted due to Doppler's effect by movement of the target object relative to the radar. The CW receiver filters out any returns without a Doppler shift, i.e., targets which are not moving with respect to the radar. When the receiver detects the presence of a Doppler shifted signal, the receiver sends a notification containing information about presence of the target object.
Another type of radar is a two-frequency CW radar. The two-frequency CW radar transmits electromagnetic energy at a first frequency and a second frequency. The transmitted energy is reflected by a target object and received by a two-frequency receiver. The receiver measures the difference between the phase of the signal received at the first frequency and the phase of the signal received at the second frequency. The distance to the target object can be calculated from the measured phase difference. Unfortunately, the two-frequency CW radar performs poorly when there are multiple target objects at different ranges, and thus the range measurement obtained from a two-frequency CW radar in the presence of multiple target objects unreliable.
There have been known FMCW radars used as vehicle-mounted radars to detect the presence of target object or obstacles, distance to a preceding vehicle, and relative speed of the preceding vehicle from the vehicle equipped with the FMCW radar.
In order to detect target characteristic such as presence of a preceding vehicle, downrange distance to the preceding vehicle, and relative speed of the preceding vehicle, the FMCW radar transmits a radar wave via a directional antenna unit. The frequency of the radar wave is modulated so as to linearly vary in time. After the target object reflects the radar wave, the reflected radar wave is received by the radar and transformed into a received signal to be subjected to signal processing for obtaining the target characteristic. The FMCW radar mixes the transmission signal and the received signal to produce a beat signal. The beat signal is subjected to a frequency analysis, for example, a fast Fourier transformation (FFT) and the like, to obtain the peak frequencies of the beat signal (beat frequencies) from which the distance to the target object and the relative speed between the FMCW radar and the target object can be determined. The frequency spectrum has peak intensities in the intensity versus frequency characteristic curves. The beat frequencies have the peak intensities.
During those operations, there is a possibility that the FMCW radar receives not only the reflected wave from the target object, but also a radar wave transmitted from some other radar installed in another vehicle, such as a vehicle running on the same or other side of the road (e.g., a preceding vehicle or an oncoming vehicle). That is, interference between the FMCW radar with which the subject vehicle is equipped and the other radar installed in the other vehicle may occur. As a result of interference, it is hard to detect the beat frequencies accurately, and the distance to the target object such as the preceding vehicle or the relative speed of the target object cannot be accurately detected.
One of the reasons for difficulties in detecting such target characteristic accurately is that frequency spectrum characteristic of the beat signal contains a broad peak. The broad peak in the frequency spectrum characteristic of the beat signal may be caused by interference which occurs in cases where the FMCW radar and the other radar have different modulation gradients of radar waves from each other (even if only slightly), or where the other radar is not FMCW type, for example, but two-frequency continuous wave, multi-frequency continuous wave, pulse, spread spectrum, and the like. The broad peak in the frequency spectrum characteristic may raise the noise floor level of the frequency spectrum characteristic of the beat signal so that the peak height of peak frequency of the beat signal (beat frequency) generated by mixing of the transmission signal and the received signal does hot exceed the noise floor level. In general, the noise floor level is the intensity of the noise from unidentified sources. As a result, the peak frequency cannot be detected accurately for the beat frequency. This results in an inaccurate detection of the target characteristic. That is, the distance to the target object or the relative speed of the target object may be erroneously determined.
In Japanese Published Patent Application No. 2006-220624 and the corresponding U.S. Patent Publication No. 2006/0181448, Natsume et al. discloses an FMCW radar which is capable of determining whether or not the FMCW radar is interfered with by some other radar.
The FMCW radar of Natsume et al. extracts high frequency components larger than a threshold frequency below which the beat frequency corresponding to the target characteristic of a target object located within the measuring rage of the FMCW radar should be positioned from the full frequency components of the beat signal. A high frequency range is defined as a frequency range containing frequency components exceeding the threshold frequency. Intensities of high frequency components of beat signal are used to calculate a reference value which is considered to relate to background noise or noise floor level. Then it is determined whether or not the FMCW radar is interfered with by some other radar based on the calculated reference value. In one of the embodiments of the FMCW radar of Natsume et al., the reference value is a sum (integral) of the intensities of the frequency components over the high frequency range. A determination whether or not interference between the FMCW radar and some other radar occurs is performed based on the sum of the intensities of the high frequency components. In another embodiment of the FMCW radar of Natsume et al., the reference value is a number of frequency components which satisfy predetermined conditions. The predetermined conditions are those that are beyond a predetermined frequency threshold and the intensities of the frequency components exceed a predetermined intensity threshold, wherein the predetermined frequency threshold is set to be out of a range within which the beat frequency corresponding to the target object located in the measuring range should be positioned, and the predetermined intensity threshold is set to be a sufficiently large value which cannot be obtained without occurrence of interference by some other radar. The predetermined frequency threshold can be set to twice the threshold frequency. It is judged whether or not interference between the FMCW radar and some other radar occurs based on the number of frequency components which satisfy the above-mentioned predetermined conditions.
The fundamental fact that is utilized by the conventional FMCW radars including that of Natsume et al. in detection of interference between the FMCW radar and some other radar is that an increase of the noise floor level of the frequency spectrum characteristic of the beat signal increases the sum of intensities of the high frequency components and increases the number of frequency components which satisfy the predetermined conditions. Using this fact, if the sum or the number exceeds corresponding threshold value, the conventional FMCW radars conclude that interference between the FMCW radar and some other radar is present.
However, the sum and the number just mentioned are increased by presence of some large or long object located far beyond the measuring region of the FMCW radar. Such a large or long object produces a beat signal having a higher beat frequency than that corresponding to the target object located in the measuring range. In particular, if there are more than a few target objects, a broad peak in the high frequency region of the frequency spectrum characteristic can appear, and may enhance the sum of intensities of the high frequency components or increase the number of the frequency components which satisfy the predetermined conditions beyond the corresponding threshold values. Hence, the conventional FMCW radars using the above mentioned fact may erroneously detect interference due to the existence of large or long target objects located far beyond the measuring region of the FMCW radar.
Further, if there are some large vehicles such as trucks and lorries, or large and long buildings such as a freeway bridge and its piers, the frequency spectrum characteristic of a beat signal may contain multiple high intensity peaks in the high frequency region.
Thus, large target objects located far beyond the measuring region of the FMCW radar enhance the sum of intensities of the high frequency components and increase the number of frequency components which satisfy the predetermined conditions even if there are no other radars near, and result in erroneous determination of occurrence of interference between the FMCW radar and some other radar. This means that it is necessary to establish a method for the FMCW radar for accurately detecting noise floor level in order to reliably detect the presence or absence of large target objects located far beyond the measuring region of the FMCW radar. Further, it is necessary to establish a method for FMCW radar for accurately determining whether interference between the FMCW radar and some other radar occurs even if some large or long target object such as trucks and lorries, or large and long buildings such as a freeway bridge and its piers exists beyond the measuring region of the FMCW radar.
The above-mentioned difficulties pose a problem: how the noise floor level should be estimated accurately based on incident wave to the receiving antennas of the radar.
In a prior method for a radar system that transmits a radar wave and receives the reflected radar wave by a target object to detect the target characteristic such as the downrange distance between the target object and the radar system for estimating noise floor level of a beat signal generated by mixing the radar wave and the reflected radar wave, a functional value of the maximum power spectrum of the beat signal has been recognized as noise floor level. Komori et al. disclose in WO 2006/120824 a method for determining the noise floor level as a function of the maximum power spectrum of the beat signal. In the method of Komori et al., if any spike noise is detected, the noise floor level of the frequency spectrum characteristic of the beat signal is determined based on the maximum absolute value of the spike noise. In this method, it is necessary to predetermine accurately the relationships between the maximum absolute value of the spike noise and the noise floor level of the frequency spectrum characteristic of the beat signal. This determination may be a difficult task if any interference between the radar and some other radar occurs.
In Japanese Published Patent Application No. 2004-163340 and the corresponding U.S. Patent Publication No. 2004/0095269, Uehara et al. disclose a vehicle-mounted radar system that detects reception of interference wave and estimates noise floor level. The radar system disclosed by Uehara et al. comprises a transmitting means for transmitting an electromagnetic wave and a receiving means for receiving the electromagnetic wave reflected by a target object. The radar system of Uehara et al. further comprises a signal processing means for measuring a distance between the radar system and the target object and a relative velocity on the basis of the transmitted electromagnetic wave and the received electromagnetic wave, and an interference detecting means for suspending a transmit operation of the transmitting means under a control of the signal processing means to detect an interference signal from an other external device. With this structure, because only noise signals such as interference wave entering the radar system are measured without measuring the reflected wave of any obstacles, the noise floor level can be calculated according to the definition of the noise floor level. However, it is necessary to suspend the transmit operation to estimate the noise floor level and to detect occurrence of interference. This means that during noise floor level estimation and interference detection, any target characteristic such as presence of a target object within the measuring range of the radar system, distance between the radar system and the target object, and relative velocity of the target object to the radar system can not be determined. This means that a continuous measurement of target characteristic can not performed.
Therefore, it is desired a radar that is capable of estimating noise floor level accurately, detecting occurrence of interference between the radar and some other radar reliably, and measuring target characteristic such as presence of a target object within the measuring range of the radar system, distance between the radar system and the target object, and relative velocity of the target object to the radar system accurately, even if some large or long target object such as trucks and lorries, or large and long buildings such as a freeway bridge and its piers exists beyond the measuring region of the radar, and even if there are multiple target objects within the measuring range of the radar.